SEOUL, South Korea -- North Korea notified rival South Korea on Tuesday that it planned a live-fire drill near the countries' disputed western sea boundary, a possible indication of rising frustration in Pyongyang as it unsuccessfully pushes for outside aid.

South Korean Defence Ministry spokesman Kim Min-seok told reporters that military officials would closely monitor the drills. Last month, South Korea fired artillery shells into the North's waters after North Korean shells from a live-fire drill landed south of the disputed sea boundary.

Both Koreas regularly conduct artillery drills in the Yellow Sea. Analysts have said that Pyongyang's announcements of live-fire drill plans are an expression of frustration by the impoverished country at making little progress in a recent push to win outside aid.

The drills are sensitive because they are staged near a disputed maritime line separating the countries. The so-called Northern Limit Line was drawn by the U.S.-led U.N. Command without Pyongyang's consent at the close of the 1950-53 Korean War, which ended with a truce, not a peace treaty, leaving the peninsula still technically in a state of war. North Korea routinely argues that the line should run farther south.

A year after threatening each other with war, the Koreas had restored some trust and held reunions earlier this year of families divided by the war. But ties have since steadily soured. Pyongyang this week launched a sexist rhetorical attack on President Park Geun-hye, calling her to a "despicable prostitute."

North Korea test-fired two medium-range ballistic missiles in March into the Sea of Japan. Pyongyang had earlier staged a series of shorter-range rocket launches to protest annual springtime military drills by the U.S. and South Korea that Pyongyang says are invasion preparation.

South Korean officials have also warned the North could be preparing for its fourth nuclear test.

The North's announcement over live-fire drills comes as South Korea deals with the tragedy of a ferry sinking that has left more than 300 dead or missing.